1. Field of Invention
This invention relates to heat engines similar in design to earlier "hot air" engines. These engines produce a motive power from compressed air. A compressor section produces compressed air to be combusted, with a suitable fuel, externally of a separate positive displacement expansion device.
2. Discussion of Prior Art
The term "heat engine" is used to describe a machine which produces useful work from the combustion of a flammable fuel and air mixture. Among the most successful are the modern gasoline fueled two and four stroke spark-ignition engine, the two and four stroke oil fueled compression-ignition engine, and the gas turbine engine.
This invention more closely resembles the past "hot-air" engines. These designs utilize combustion prior and external to the power producing expansion device, such as a reciprocating piston or rotating turbine wheel. Many of the components of this invention have been included in previous patents, but the lack of several key elements as well as the lack of a proper conceptual framework has prevented each from achieving significant commercial success.
One of the earliest, and most successful, of the related works is shown in U.S. Pat. No. 125,166 to Brayton (1872). His engine consists of integral compression and expansion pistons within a common cylinder body. Fuel and air are mixed prior to compression, producing a potentially dangerous, pressurized mixture. The ratio of compression to expansion was fixed, therefore, the engine is more suitable for a constant power application. Due to the limitations of controls and materials of the day, as well as the competition from the internal combustion engine, Brayton's gas engine disappeared from use. Nevertheless, the power cycle he pioneered, in which combustion theoretically occurs at a constant pressure, still bears his name.
Due to the petroleum shortages in the 1970's, as well as an increasing problem of automotive air pollution, there was a resurgence of interest in the "external combustion" designs. One such design is described by U.S. Pat. No. 3,775,973 to Hudson (1973). Unfortunately, it lacks a compressed air accumulator, and becomes overly complicated with its two stage, compound compression and expansion pistons.
U.S. Pat. No. 3,811,271 to Sprain (1974) and U.S. Pat. No. 3,839,858 to Avermaete (1974) each illustrate an improved powerplant. Both suffer from a few of the same major disadvantages of earlier attempts. One such problem is an inadequately sized, or absent, compressed air accumulator. Another is an integral compression and expansion engine cylinder block which transfers the heat of combustion to the induction components, reducing volumetric efficiency, or efficiency of air flow.
A further improved engine is described by U.S. Pat. No. 4,149,370 to Vargas (1979). Even though Vargas includes more of the essential components, his engine cools the compressed air charge during storage in the accumulator, reducing cycle efficiency. Further, he states that temperatures will be maintained at levels low enough to facilitate the use of conventional materials. This necessitates an undesirable engine cooling system.
The most advanced design was put forth in U.S. Pat. No. 4,653,269 to Johnson (1987). His design includes the complication of a variable transmission between the compression and expansion devices. Also, in addition to being undersized, the accumulator is not permanently installed in the fluid flow path, but is selectively connected upon demand.
In all of the above designs, no provisions are made to accommodate the high temperature combustion products which will be in contact with the valves, pistons, and cylinders. The use of conventional materials necessitates the cooling of various engine components and often the compressed air supply. Any heat removed must only be reintroduced during the combustion process, thereby lowering the thermodynamic cycle efficiency. And lastly, while several of the above designs incorporate a compressed air accumulator, they are undersized and fail to take full advantage of the torque reserve and operating characteristics of a properly sized unit.
The last reference to be cited is SAE technical paper number 970068 by Leidel (1997). The paper was written by this inventor for the purpose of desribing an inceptive study which produced this invention. It also includes research and discussion of material and tribological issues.